How Cheeky: Fossil Fish Is Oldest Creature With a Face

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A newly discovered fish fossil is the earliest known creature
with what might be recognized as a face.

Entelognathus primordialiswas an
ancient fish that lived about 419 million years ago in the
Late Silurian seas of China. The finding, detailed today (Sept.
25) in the journal Nature, provides a link between two groups of
fishes previously thought to be unrelated, challenging long-held
notions of how vertebrate faces evolved.

Until recently, scientists assumed the common ancestor of
gnathostomes was more similar to cartilaginous fish. This
ancestor "would have looked something like a shark, devoid of
armor and with a largely cartilaginous skull," said study leader
Min Zhu, a paleontologist at the Institute of Vertebrate
Paleontology and Paleoanthropology, in Beijing.

Bony goggles

To investigate, Zhu and her colleagues examined an
Entelognathusfossil found in the remains of an ancient
seabed known as the Kuanti Formation, near the town of Qujing, in
Yunnan, southern China.

Analyses showed Entelognathuswas nearly 8 inches (20
centimeters) long, with a heavily armored head and trunk and a
scaly tail. Its eyes were tiny and set inside large "bony
goggles," the researchers report. And despite having a jaw, it
had no teeth.

All known bony fishes have teeth, so the fossil adds to the
mystery of how teeth originated, Young said.

Next, they compared the fossil with known jawed vertebrates,
across a suite of physical characteristics. They also examined
the facial bones in detail using an imaging technique called
X-ray micro-computerized tomography.

At first blush, Entelognathusappears to be an ordinary
placoderm, an extinct type of
heavily armored fish. All placoderms discovered before now
have sported simple jaws and cheeks, with only a few large bones
making up their outer surfaces.

"But the original fossil of Entelognathus proved to be
something far more bizarre and significant," Zhu told
LiveScience.

Fish face!

On closer inspection, the fish possesses a complex arrangement of
smaller bones known as a premaxilla and maxilla on its upper jaw,
a dentary, or mandible, on its lower jaw, as well as cheekbones.
These complex facial bones are characteristic of bony fish and
land animals, including humans, making this bizarre-looking fish
the most ancient animal with what humans would recognize as a
face, Zhu said.

"This is certainly an amazing discovery, both because of its age
[Silurian] and the morphology of the lower jaw," Gavin Young, a
zoologist at Australian National University in Canberra who was
not involved in the study, told LiveScience.

Scientists have long assumed placoderms and bony fish were
unrelated, and that bony fish evolved their face bones from
scratch. But the new finding suggests bony fish inherited their
skulls from their placoderm ancestors, Zhu said.

The researchers further suggest that another extinct fish group
called acanthodians, which resembled
small sharks with big, bony fin-spines, actually belong
to the same lineage as cartilaginous fishes, andprogressively
lost their armored plates.

The discovery "throws a spanner" in the works of
some long-held ideas about vertebrate evolution," Zhu said
of the fish with a face.